Voltage Build-Up of a Shunt Generator

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Voltage Build-Up of a Shunt Generator


Before loading a shunt generator, it is allowed to build up its voltage. Usually, there is always present some residual magnetism in the poles, hence a small e.m.f. is produced initially. This e.m.f. circulates a small current in the field circuit which increases the pole flux (provided field circuit is properly connected to armature, otherwise this current may wipe off the residual magnetism). When flux is increased, generated e.m.f. is increased which further increases the flux and so on.

 As shown in Fig. 3, Oa is the induced e.m.f. due to residual magnetism which appears across the field circuit and causes a field current Ob to flow. These current aids residual flux and hence produces, a larger induced e.m.f. Oc. In turn, this increased e.m.f. Oc causes an even larger current Od which creates more flux for a still larger e.m.f. and so on.

Now, the generated e.m.f. in the armature has (a) to supply the ohmic drop 𝑖𝑓𝑅𝑓 in the winding and (b) to overcome the opposing selfinduced e.m.f. in the field coil i.e. 𝐿 𝑑𝑖𝑓 𝑑𝑑 because field coils have appreciable self-inductance. 𝐸𝑔 = 𝑖𝑓𝑅𝑓 + 𝐿 𝑑𝑖𝑓/ 𝑑𝑑  If (and so long as), the generated e.m.f. is more than the ohmic drop 𝑖𝑓𝑅𝑓, energy would continue being stored in the pole fields.

 For example, as shown in Fig. 3, corresponding to field current OA, the generated e.m.f. is AC. Out of this, AB goes to supply ohmic drop If Rsh and BC goes to overcome  self-induced e.m.f. in the coil. Corresponding to If = OF, whole of the generated e.m.f. is used to overcome the ohmic drop.

 None is left to overcome 𝐿 𝑑𝑖𝑓 𝑑𝑑 ⁄ . Hence no energy is stored in the pole fields. Consequently, there is no further increase in pole flux and the generated e.m.f. With the given shunt field resistance represented by line OP, the maximum voltage to which the machine will build up is OE. If resistance is decreased, it will build up to a somewhat higher voltage. OR represents the resistance known as critical resistance. If shunt field resistance is greater than this value, the generator will fail to excite.


Conditions for Voltage Build-Up of a Shunt Generator

 The necessary conditions for voltage build-up in a shunt generator are:

 (i) There must be some residual magnetism in generator poles. 

(ii) The connections of the field winding should be such that the field current strengthens the residual magnetism.

 (iii) The resistance of the field circuit should be less than the critical resistance. In other words, the speed of the generator should be higher than the critical speed.



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